The dynamics of librational motion in N-type crystals ( alpha -N//2, alpha -CO, N//2O, CO//2) is treated by taking into account both anharmonic and correlation effects. The method used is similar to the Tyablikov's method in the theory of magnetism. The main thermodynamic characteristics of the librational subsystem are calculated: the order parameter, rms librational angle, librational mode frequencies and corresponding Grueneisen parameters, librational heat capacity, internal and free energies. The librational isotope effects for alpha - **1**4N//2 and alpha - **1**5N//2 is considered. An explanation of the anomaly isotope effects in heat capacity is proposed. A theory of the phase transition into orientationally disordered state is developed.